Again EMC 20% Guarantee
The Prince of Dataness
EMC is offering a guarantee with its unified storage arrays, saying you will need 20 per cent less raw capacity than from other suppliers.
You've got to use EMC best practices and there's no direct performance guarantee. Instead EMC states: "This translates to superior storage efficiency when compared to other unified storage arrays — even those utilizing their own documented best practices."
EMC says that if its storage products are not more efficient by 20 per cent it will match the shortfall.
And again they added a great short movie on youtube.